A) Field of the Invention
The present invention relates to a ZnO based semiconductor device and its manufacture method.
B) Description of the Related Art
Zinc oxide (ZnO) is semiconductor of a direct transition type having a band gap energy of 3.37 eve at a room temperature, and has characteristics that a binding energy of exciton is 60 meV which is higher than that of other semiconductors, source materials are inexpensive, and there is less adverse effects upon environments and human bodies. It is therefore expected to realize a semiconductor light emitting device made of ZnO and having a high efficiency and a low consumption power.
It is, however, known that the electric characteristics, optical emission characteristics and the like of a ZnO based semiconductor light emitting device are degraded considerably by moisture adsorption to the device surface. It is very important industrial technologies to prevent moisture adsorption to a device surface, maintain stable device characteristics and prolong a device lifetime.
JP-A-2004-247654 discloses a technique of covering the surface of a ZnO based semiconductor light emitting device with a multilayer protective film made of two or more stacked different insulating organic compound films (e.g., polycarbonate and polyimide), in order to mitigate a problem that reactive current flows on the device surface via conductive material reduced by moisture adsorption and a problem of an increase in defects to be caused by device temperature rise due to optical emission and high density optical energy exposure. It also discloses a technique of improving tight adhesion by inserting an inorganic protective film (e.g., an SiO2 film) between such a multilayer protective film and device.
In order to mitigate similar problems to those described in JP-A-2004-247654, JP-A-2004-266057 discloses a technique of using metal oxide or nitride as a protective film providing the protective effects relative to a reducing atmosphere. It is also describes that it is efficient that an insulating organic film is disposed on the surface of an uppermost layer.
A variety of techniques capable of improving moisture-proof of a ZnO based semiconductor device have been desired.